This invention relates to a method of toughening a glass sheet by heating the glass sheet to a temperature above the strain point and quenching the heated glass sheet with blasts of cool air. The method is particularly suitable for use in toughening a relatively thin glass sheet, viz. A glass sheet about 1.5-3.0 mm in thickness, which may be a curved glass sheet for use as an automobile window glass.
As recent automobiles are designed to reduce the gross weight, there is a growing demand for toughening of relatively thin glass sheets for use in automobile side and rear windows. To ensure an adequate margin of safety for the drivers and passengers in case of breakage of automobile window glass, there are official regulations which specify fracture requirements of toughened glass sheets. Typical regulations require that the number of glass fragments contained in any 5 cm .times.5 cm square traced on the glass sheet (excluding a circular area with a radius of 7.5 cm around the point of impact and marginal areas 3 cm in breadth) should fall in the range from 60 to 400, that the fragments should not be larger than 3 cm.sup.2 in surface area and that the fragments should not include elongated particles longer than 75 mm (such elongated particles are called "splines").
However, it is not easy to toughen glass sheets thinner than about 3 mm by a conventional air quenching method so as to fully meet the official regulations. In general, the quenching is for producing a center-to-surface gradient of temperature through the thickness of the glass sheet and results in permanent compressive stresses being produced in the surface layers of the glass sheet with compensating tensile stresses in the center of the glass thickness. In the case of a thin glass sheet, it is difficult to create and maintain a suitable gradient of temperature in the glass sheet during the quenching process. The difficulty is augmented when the thin glass sheet is an intricately curved glass sheet.
For toughening relatively thin glass sheets by air quenching, there are some proposals with a view to enhancing cooling efficiency. For example U.S. Pat. No. 4,578,102 proposes directing jets of a mixture of air and atomized water onto the heated glass surfaces by means of Laval nozzles. Air is supplied to the Lavel nozzles at such a pressure that the jet velocity at the exit of each nozzle becomes at least sonic, while water is introduced from a radial direction into the constricted throat section of each nozzle. The mixture of air and atomized water has a higher specific heat than air, and it is intended to rapidly extract heat from the glass sheet surfaces by using two-phase jets high in both velocity and specific heat. However, from a practical point of view, the use of water besides air offers complicacy, and very high precision equipment is required for complete atomization of water and thorough mixing of atomized water with air during the passage of the two fluids from the nozzle throat to the nozzle exit. Additionally, there is a possibility that droplets of water will hit the heated glass sheet to cause breakage of the glass sheet.
JP-A 60-145921 relates to quenching of a heated glass sheet with air jets and proposes to determine the air pressure and the nozzle configuration such that the maximum drop of the cooling air pressure takes place at the exit of each nozzle and the air jet velocity at the nozzle exit becomes sonic. It is a disadvantage of this method that fluctuations of the air supply pressure in the quenching equipment are liable to be transmitted to the glass sheet surfaces so that the glass sheet under quenching, is liable to be distorted in when the glass sheet is relatively thin. Besides, in this method it will be necessary to give very careful consideration to the arrangement of the quenching nozzles.
U.S. Pat. No. 4,735,646 relates to quenching of a heated glass sheet and proposes to produce a shock wave in the air chamber, in each of two opposite blast heads from which nozzles protrude, by forcing compressed air to rapidly expand in each air chamber such that the pressure (gauge pressure) of air rapidly drops from a predetermined first pressure ranging from 2 to 8 kg/cm.sup.2 to a predetermined second pressure ranging from 0.05 to 0.5 kg/cm.sup.2. By virtue of the propagation of the shock wave through the air chamber and the nozzles, the air jets have high kinetic energy at the moment of impingement on the glass surfaces and hence are high in the initial cooling effect. By this method, even glass sheets thinner than 3 mm can be toughened so as to meet the regulations for toughened glass sheets for use in automobile windows. However, this method requires an air quenching apparatus of relatively large capacity. JP-A 64-3029 relates to toughening of a glass sheet by using the method of U.S. Pat. No. 4,735,646 and proposes to first quench a central region of the heated glass sheet and then gradually direct the quenching air jets toward the edges of the glass sheet. This proposal is for relatively mildly tempering glass sheets about 3-5 mm in thickness and is not suitable for sufficiently toughening thin glass sheets for use in automobile windows.
There is another problem relating to the toughening of a thin glass sheet in that it is intricately curved. During quenching of the heated glass sheet in a suspended state, the glass sheet is liable to sway under the action of blasts of cooling air. The swaying of the hot glass sheet is unfavorable for accurate toughening of the glass sheet and, further, is liable to cause some deformation of the glass sheet, or breaking, in an extreme case, or mechanical or optical distortion of the toughened glass sheet.